An airport runway approach light is used to assist aircrafts in landing on a runway. A typical runway approach light source is a thermal source such as an incandescent bulb and a halogen lamp. The thermal source emits infrared spectral components in addition to components of a visible light spectrum. When the weather condition is favorable and the visibility is good, a pilot during landing uses the components of the visible spectrum with naked eyes while viewing the runway approach lights. However, when the weather condition is unfavorable or the visibility is bad, a pilot uses an infrared (IR) camera or an enhanced flight vision system (EFVS) equipped in the cockpit to detect infrared rays emitted from the thermal source for safe landing.
The power consumption of a thermal source used for runway approach light is very large (around 100˜500 W). Therefore, the thermal source is extremely inefficient because the quantum efficiency to convert energy from an input electrical power to an optical power is very low, and the beam shaping mechanism to convert a generated beam pattern to a required beam pattern is difficult resulting in a high coupling loss.
An inefficient beam shaping mechanism is inherent for a thermal source because the emitted light output from the thermal source is isotropical. As a result, the thermal source needs to use a lens to convert and match an omni-directional output light to a required beam pattern, for example, converting a 360 degree isotropic beam pattern to an approximately 8 degree at Full Width at Half Maximum (FWHM). Among colors of a visible light spectrum, a green threshold light used for an airport runway approach light requires a green filter, thus its efficiency is even worse.
As a result, a need for an approach light with an energy efficient light emitting diode (LED) light source has been raised recently. However, an energy-efficient LED (e.g., a steady burning white light LED and a threshold green light LED) has, in a practical situation, only a visible spectrum and e its almost no infrared wavelengths. As such, an LED light source is not appropriate as a runway approach light for a next generation airport system because it cannot be safely used during severe or low visibility weather conditions.
To solve this issue, attempts have been made to find an efficient infrared (IR) light source to be used in an airport runway approach light, For example, a graphite foam requires heating the material at about 600 to 800 degrees Celsius to generate an infrared light. However, an athermalization material such as a graphite foam is not reliable and requires a high temperature insulation in a lamp housing.